27 Update
Group A Streptococcus
and its antibiotic resistance
Lo Strepotococco beta-emolitico di gruppo A e la sua resistenza
alla terapia antibiotica
D. PassÀli, M. lauriello1, G.C. PassÀli2, F.M. PassÀli, l. Bellussi
eNT Clinic, university of siena; 1 experimental Medicine Department,university of l’aquila; 2 institute of
otolaryngol-ogy, Catholic university of the sacred Heart of rome, italy
Summary
acute pharyngo-tonsillitis caused by beta-haemolytic group a Streptococcus is a common disease in childhood. Epithelial cells are the initial sites of the host invasion by group a Streptococcus. although group a Streptococcus has been considered an extracellular pathogen, recent studies have demonstrated that strains of this bacterium can internalize into epithelial cells both
in vitro and in vivo. As adherence to and internalization into host cells significantly contributes to the pathogenesis of group
a Streptococcus infections, internalization of group a Streptococcus by human epithelial cells has been extensively studied during the past decade. multiple mechanisms are involved in this process. most strains of Streptococcus pyogenes express the fibronectin-binding proteins F1 and F2, which promote bacterial adherence to and entry into human cells. Strains containing the gene for the protein F1 have been proved to be responsible for the failure of antibiotic treatment to eradicate Streptococcus
pyo-genes. Thus, in a significant number of cases, streptococcal internalization might contribute to eradication failure and persistent
throat carriage. Since treatment failure, asymptomatic group a Streptococcus carriers and recurrent group a Streptococcus infections represent the main group A Streptococcus reservoir, from which the bacteria are spread in the general population, the choice of antibiotic is crucial. Beta-lactams select a large number of F1-positive organisms: therefore, macrolides, and, pos-sibly, last generation molecules, are the best and first choice for antibiotic treatment against group A Streptococcus.
Key wordS: Pharyngo-tonsillitis • Group a Streptococcus internalization • antibiotic resistance • Beta-lactams resistance • macrolides
riaSSunto
La faringo-tonsillite acuta causata dallo Streptococco beta-emolitico di gruppo A è una patologia comune nell’età infantile. Le cellule epiteliali sono il sito iniziale di invasione da parte dello Streptococco, che viene classicamente considerato un pa-togeno extracellulare. Recenti studi hanno però dimostrato che alcuni ceppi di questo batterio sono in grado di internalizzarsi all’interno delle cellule epiteliali sia in vitro che in vivo. Dal momento che sia l’aderenza che l’internalizzazione nelle cellule dell’ospite contribuiscono in modo significativo alla patogenesi delle infezioni da Streptococco beta-emolitico di gruppo A, l’internalizzazione di quest’ultimo è stata oggetto di numerosi studi nell’ultima decade. Molteplici meccanismi sono implicati in questo complesso processo. Diversi ceppi di Streptococcus pyogenes esprimono le fibronectin-binding proteins (proteine leganti la fibronectina) F1 e F2, le quali promuovono l’aderenza e l’ingresso dei batteri nelle cellule umane. È stato dimo-strato che i ceppi dotati del gene per la proteina F1 sono responsabili del fallimento del trattamento antibiotico. Quindi in un numero significativo di casi l’internalizzazione dello Streptococco potrebbe contribuire al fallimento della eradicazione del germe e allo stato di portatore sano dello stesso in faringe. Dal momento che il fallimento terapeutico, lo stato di portatore asintomatico e le infezioni ricorrenti da Streptococco rappresentano la principale riserva dalla quale i batteri vengono diffusi all’interno della popolazione, la scelta dell’antibiotico è cruciale. Gli antibiotici beta-lattamici selezionano un gran numero di microrganismi F1-positivi, pertanto i macrolidi e possibilmente le molecole di ultima generazione, sono la prima e la migliore scelta per la terapia antibiotica contro lo Streptococco beta-emolitico di gruppo A.
PArole chiAve: Faringo-tonsilliti • Streptococco Gruppo A • Internalizzazione batterica • Antibiotico-resistenza • Resistenza ai Beta-lattamici • Macrolidi
Introduction
Streptococcus pyogenes (S. pyogenes) (group a
Streptococ-cus (GaS)) is exclusively a human pathogen, the t anti-gens of which form the basis of a major serological typing
scheme, an alternative or supplement to m typing. GaS is well known as a highly adhesive extra-cellular organism, the virulence of which is related to the production of exo-toxins and the presence of particular surface components. its extraordinary biological diversity becomes evident in the
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wide range of diseases and in the antigenic heterogeneity present on its surface. as a prerequisite for colonization or causing local infections, S. pyogenes needs to adhere to eu-karyotic cell surfaces. recently, the capacity of S. pyogenes to invade eukaryotic cells has been reported. A better under-standing of the surface proteins involved in the interaction between group A Streptococci and epithelial cells will be helpful in the development of new strategies to fight infec-tions. in fact, antibiotic treatment has been demonstrated to be unable to eradicate group-A Streptococci in up to 30% of patients with pharyngotonsillitis 1. thus an antibiotic,
tar-geted against the strains of group-a Streptococci that have entered into the respiratory epithelial cells, which is unable to penetrate the cell membrane (such as penicillins), would fail to eradicate the bacteria.
in the present review, an attempt is made to answer the fol-lowing questions, referring to the clinical concern regarding gAS resistance to antibiotics:
1. how can S. pyogenes internalize into human cells? 2. what is the fate of S. pyogenes following
internaliza-tion?
3. why do antibiotics fail in the treatment of group A Strep-tococcus infections?
How can Streptococcus pyogenes
internalize into human cells?
Bacterial pathogens have evolved various strategies for their survival. S. pyogenes produces several proteins that influence host-pathogen interactions.
in the adherence process, the host extracellular matrix (ECm) proteins act as prime targets. GaS may express several microbial surface components recognizing adhesive matrix molecules, which attach to fibronectin (Fn), a matrix protein, or collagen. Fn was discovered by harvard’s Plas-ma Protein Program as plasPlas-ma “cold-insoluble globulin”, in the 1940s. it is able to change shape in response to various environmental conditions and interactions with other sub-stances found in the extra-cellular space.
Fn-binding proteins are known as mediators for adherence of Streptococci to eukaryotic cells and play an important role in the adherence of S. pyogenes to human epithelial cells. The Fn-binding proteins F1 and F2, which are con-sidered to be major group A streptococcal virulence factors, are functionally related but, structurally dissimilar.
the interaction of S. pyogenes Fn-binding protein (Sfbi = F1) with Fn on epithelial cells triggers bacterial internali-zation 2. Blocking of the Sfbi adhesin by either antibodies
against the whole protein or antibodies against the Fn-bind-ing domains of Sfbi, prevents both S. pyogenes attachment and internalization. inert latex beads, pre-coated with the purified Sfbi protein, are ingested by eukaryotic cells, dem-onstrating that Sfbi is sufficient to trigger the internalization process.
entry of serotype M1 S. pyogenes into host cells depends on binding of the host glycoprotein Fn by the bacterial M1 protein. Deletion of both the n-terminal a and B repeats regions of M1 abrogated Fn binding and intracellular in-vasion. deletion of either the A domain (M1deltaA) or B repeats (M1deltaB) significantly reduced, but did not com-pletely eliminate Fn binding 3.
Both the surface proteins F1and M6 are required for
effi-cient internalization: the protein F1 mediates streptococcal internalization and the M6 protein is required for more ef-ficient entry of the bacterium 4 5. the functional upstream
domain (FUd) of S. pyogenes (protein F1) interacts with the amino-terminal type i modules of Fn to unveil the cell-binding region of Fn 6.
rocha and Fischetti 7 identified a new S. pyogenes
Fn-bind-ing protein (PFBP). This 127.4 kda protein shows a struc-ture characteristic of cell wall-associated proteins in gram-positive organisms. its gene (pfbp) is transcribed during cell growth. The amino acid sequence exhibits a variable n-ter-minal region (105 amino acid with no homology with pre-viously described Fn-binding molecules) and a conserved C-terminal region.
Brandt et al. 8 carried out the characterization of 40
consecu-tive S. pyogenes isolated from 18 patients with pharyngitis and bacteriological treatment failure. they reserved a spe-cial reference to prtF1 (the gene encoding the Fn-binding protein F1) and sic (streptococcal inhibitor of complement)/ drs (distantly related sic).
on the basis of their data, the authors suggest that neither the presence of prtF1 nor the diversification of sic/drs is required for the persistence of S. pyogenes in pharyngitis. gAS strains differ in the genetic potential to express Fn binding proteins Sfbi, Sfbii and PrtF2: strains possessing two or more of the genes for these FBPs bound Fn significantly more than strains possessing none or one of the genes 9. as no
correlation was found between Fn binding ability and the avidity of the strains to adhere to epithelial cells, the authors suggest that while Fn binding is important for adhesion, for many gAS strains, the extent of Fn binding is not the criti-cal determinant of adherence. Further studies came to other conclusions. the spe B (streptococcal pyogenic exotoxin B) is a secreted cysteine protease erythrogenic toxin. inactiva-tion of the spe B gene enhanced Fn-dependent uptake of the pathogens compared to that in the isogenic wild-type strain. The effect was attributed to an abrogation of Fn binding to the surface of the bacteria, not involving the streptococcal Fn-binding protein Sfbi 10 11. The spe B protease specifically
alters bacterial cell surface proteins and, thereby, influences pathogen uptake. The influence of group A streptococ-cal acid glycoprotein (SAgP) on the expression of major virulence factors and internalization by epithelial cells had been investigated. SAg was studied using an isogenic mu-tant containing an in-frame deletion within the sagp gene 12.
The sagp mutant displayed slower growth-rate and 5-fold higher internalization efficiency than the parent strain. A down-regulation of the spe B was the most striking effect of the sagp mutation. When the authors treated the SaGP mutant cells with the exogenous mspe B (mature protease), their susceptibility to internalization was only partially reduced. overall, results show that SAgP modulates the expression of spe B, but also of other genes that facilitate
S. pyogenes internalization. the cysteine proteinase spe B
and the protein F1 played a correlate role in streptococcal entry into human cells 13. in fact, the protein F1 was
de-graded by spe B also when in complex with Fn. Therefore, the removal of protein F1 from the bacterial surface reduced internalization. in conclusion, spe B modulates Fn depend-ent internalization of S. pyogenes by efficidepend-ent proteolysis of cell-wall-anchored protein F1. Fn, bound to F1/Sfbi, acts as a bridging molecule towards host cell integrins, which, in turn, initiate the uptake process that leads to gAS
inter-29
nalization. in their intracellular niche, GaS can persist, pro-tected from antibiotics and host defence 14.
recently, Gorton et al. 15 investigated an association
be-tween the presence of prtF1 and prtF2 genes (the genes of proteins F1 and F2, respectively) and internalization effi-ciency in group A streptococci isolated from patients with invasive disease. The prtF2-positive isolates internalised up to three times more efficiently than isolates that had prtF1 alone (p < 0.001), and 1.5-fold better than isolates that had neither gene. no significant association was found between internalization efficiency and presence of the prtF1 gene. According to Kreikemeyer et al. 16 protein F2 expression
act as an indispensable virulence factor for both earlier and later pathogenetic stages of gAS superficial infections. in transgenic mice lacking plasma fibronectin, nyberg et al. 17 introduced the protein F1 gene in a S. pyogenes strain
lacking this gene. Protein F1-expressing bacteria were less virulent to normal mice, but when these bacteria were used to infect mice lacking plasma Fn the virulence was partly restored. Plasma Fn bound to the bacterial surface down-regulates S. pyogenes virulence by limiting bacterial spread. From an evolutionary point of view, reducing virulence by binding Fn adds selective advantages to the bacterium 17.
ac-cording to Wang et al. 18, entry of S. pyogenes into host cells is
mediated by Fn bound to protein F1. The consequent bridge to alpha5beta1 integrins leads to cytoskeletal rearrangement and uptake of Streptococci. The Authors demonstrated that integrlinked kinase (ilK) is the universal link between in-tegrins and several bacterial pathogens: in fact, the inhibition of iLK reduces the invasion of epithelial cells.
the main adhesin genes are contained in a genotype-spe-cific pattern within the FcT region of the GaS genome. The chromosomal region, designated as FcT region, is approximately 11 to 16 kb in length and contains genes encoding a collagen-binding protein (Cpa) and the sec-ond Fn-binding protein (PrtF2 or Pfbpi). The highly re-combinatorial FcT region of the S. pyogenes genome is under strong selection for change in response to the host environment 19. nakata et al. 20 identified Msmr in the
FcT region as a positive regulator of the major Fn-bind-ing adhesin protein F2 in a serotype M49 strain. Altered levels of Fn-binding proteins affect eukaryotic cell at-tachment and internalization into human pharyngeal epi-thelial cells.
the expression of GaS mSCramms (microbial surface components recognizing adhesive matrix molecules) is un-der the control of several transcriptional regulators and two-component signal transduction systems. these mechanisms are crucial, since interference with MScrAMM function alone, or in conjunction with specific manipulations of reg-ulators, is an attractive goal for novel anti-infective strate-gies 16.
another molecule involved in adherence and internaliza-tion is the M3 protein. Strains of the M3 serotype form the second largest group isolated from patients with severe in-vasive diseases and fatal infections. Eyal et al. 21
demon-strated that the M3 protein is involved in both adherence to and internalization by epithelial cells (hep-2 cells), but it is not essential for the maturation of spe B. inhibition of S.
pyogenes internalization by spe B is not related to the
pres-ence of an intact M3 protein. Thus, other proteins which serve as targets for spe B activity play a major role in the in-ternalization process. Surface-associated M proteins show
an anti-phagocytic property. Studying phagocytosis by means of human neutrophils of wild-type S. pyogenes and strains deficient in expression of M protein and/or the M-like protein h., Staali et al. 22 demonstrated that all strains
of S. pyogenes tested were phagocytosed by human neu-trophils. however, the wild-type strain could survive inside neutrophils, whereas mutant strains were rapidly killed. The authors concluded that bacterial evasion of host defences occurs intra-cellularly and that survival inside human neu-trophils may contribute to the pathogenesis and recurrence of S. pyogenes infections.
lipoteichoic acid (lTA) was demonstrated to play a role in the interactions between S. pyogenes and host cells 23. Lta
inhibited bacterial entry into hep-2 epithelial cells, but had only a slight inhibitory effect on adherence. on the basis of confocal laser microscopy imaging and analysis, Lta might exert a cytotoxic effect that interferes with bacterial internalization. a possible target for Lta activity might be the actin cytoskeleton, which is known to be essential for bacterial uptake. The LSP, a member of the Lrai-lipoprotein family in S. pyogenes, was demonstrated to be involved in eukaryotic cell adhesion and internalization 24.
the S. pyogenes Fas two-component signal transduction system could be involved in local tissue destruction and bacterial aggressiveness towards host cells 25. in fact, the
activity of the S. pyogenes Fas-system promotes high ad-herence and internalization rates, massive cytokine gene transcription and cytokine release, host cell apoptosis via a caspase-2 activation pathway, and cytotoxicity.
Which will be the fate of Streptococcus
pyogenes following internalization?
Secretory iga (siga) against group a Streptococci inhibits streptococcal adherence to pharyngeal cells 26. however, in
case of adherence, we might ask what will the fate of gAS be following internalization.
in the attempt to answer this question Marouni and Sela 27 treated epithelial cells (hep-2) harbouring
intracel-lular bacteria with antibiotics to kill extra-celintracel-lular adherent bacteria and washed. in the absence of antibiotics, massive bacterial growth was apparent in the cell medium, accom-panied by extensive death of the epithelial cells harbouring intracellular bacteria. this result suggests that intracellular bacteria had multiplied and damaged the monolayer. When they add either cytochalasin D (an internalization inhibitor) or chloramphenicol, bacterial growth and cell injury were both prevented.
Analysis of three apoptotic markers in hep-2 cells indicated that the latter underwent apoptosis. The Authors explained the data obtained by the following model:
1. internalized bacteria can induce their own externaliza-tion into the medium by a process that requires both an intact host-cell cytoskeleton and de novo synthesis of bacterial proteins;
2. intracellular and, apparently, extra-cellular free bacteria induce apoptosis through their cytotoxic activity, and re-lease essential nutrients required for their growth. the adhesion of S. pyogenes stimulates nuclear transloca-tion of the transcriptransloca-tion factor nF-kB. however, bacterial internalization is required for a sustained nuclear transloca-tion of this transcriptransloca-tional factor 28.
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nakagawa et al. 29 demonstrated that human pharyngeal
epi-thelial hep-2 cells become apoptotic with dnA fragmenta-tion by invasion of GaS. the internalizafragmenta-tion of GaS can be followed by the induction of apoptosis, which is initiated by mitochondrial dysfunction. the mechanism of GaS-induced apoptosis is different from that GaS-induced by other intracellular invasive bacteria, e.g. Shigella and Salmonella species.
Two distinct pathways for the invasion of S. pyogenes in non-phagocytic cells are known 30. According to the first
pathway, in the invasion process Fn acts as a bridging mol-ecule and the alpha5beta1 integrin as cellular receptor. The uptake process is characterized by the generation of large membrane invaginations at the bacteria-cell interface with-out evidence of actin recruitment or cellular injury. The bac-terial cells are located into phagosomes and, 24 h after in-fection, a consistent part of the bacterial population reaches the cytoplasm. Following an alternative pathway, the up-take requires major rearrangements of cytoskeletal proteins underneath attached bacteria. Bacterial attachment stimu-lates elongation and massive recruitment of neighbouring microvilli, which surround streptococcal chains. they lead to the generation of large pseudopod-like structures, as a consequence bacteria are rapidly released and replicated in the cytoplasm.
Why do antibiotics fail in the treatment
of group A Streptococcus infections?
the failure of penicillin to eradicate Streptococci from the throat occurs in up to 35% of patients with pharyn-go-tonsillitis. Several explanations have been advanced, such as coexistence of oropharyngeal beta-lactamase-producing bacteria, interference by aerobic and anaerobic commensals, penicillin tolerance, reinfection. at present, many studies support the hypothesis that the intracellular niche may protect group a Streptococcus from penicil-lin, which does not reach high intracellular concentra-tion. gAS strains were shown to survive 4-7 days inside cultured epithelial cells and strains isolated from patients with eradication failure contained the internalization-as-sociated gene, prtF1/sfbi, in higher prevalence than the strains recovered from patients with successful eradica-tion 31. thus, internalization and intracellular survival
represent a novel explanation for penicillin eradication failure. in fact, the ability of group a Streptococci to per-sist in the throat following antibiotic therapy corresponds with their capacity to adhere to and be internalised by epithelial cells.
Studying 13 strains from asymptomatic patients with bacte-riological eradication failure and 29 from patients with bac-terial eradication, Sela et al. 32 observed that the adherence
and internalization efficiencies of strains from carriers were significantly higher.
in 355 italian paediatric pharyngitis patients, 127 (35.8%) erythromycin-resistant gAS isolates were detected 33. the
Authors found 126 of the 127 macrolide-resistant isolates serum opacity factor (sof) gene positive. thus, a strong as-sociation between macrolide resistance and the presence of sof among gAS isolates was demonstrated. The Fn-binding protein F1, which is needed to enter the respiratory epithe-lial cells, is linked to the ability of some S. pyogenes strains
to persist in the throat in spite of antibiotic therapy. neeman et al. 1 hypothesised that an intracellular reservoir of
group-a Streptococci could group-account, group-at legroup-ast in pgroup-art, for fgroup-ailure to eradicate throat carriage. they investigated the frequency of prtF1-containing strains in 67 patients with pharyngo-tonsillitis. As they found 12 (92%) of the 13 patients with symptomless carriage had prtF1-containing strains in the throat, compared with 16 (30%) of the 54 patients with suc-cessful eradication (p = 0.0001), they suggested that pro-tein-F1-mediated entry to cells is involved in the causative process of the carriage state.
according to musumeci et al. 34. the rate of S. pyogenes
strains carrying the pfbpi gene was significantly higher among asymptomatic carriers (80%) than among children with pharyngitis (53%; p < 0.05). A significant association between macrolide resistance and protein F/Sfbi (p < 0.001) was detected in both groups by the same Authors. These results confirm that the presence of the pfbpi gene can be linked to the ability of S. pyogenes to persist in the throat of asymptomatic carriers.
the bacterial population isolated from baseline pharyngeal swabs by cocuzza et al. 35 showed an overall prtF1 rate
of 33%. Following antibiotic therapy, the average level of prtF1 increased among erythromycin-resistant strains (45%). The prevalence was higher (84%) in strains belong-ing to inducible resistance phenotype (imLS) and in consti-tutive resistance phenotype (cMlS) (67%) as compared to the efflux pump resistance phenotype (M phenotype) (15%) (p < 0.001). The prevalence of the prtF1 gene was signifi-cantly lower (p = 0.04) in strains belonging to M resistance phenotype as compared to erythromycin-susceptible strains (28%).
The association of prtF1with macrolide resistance and with bacteriological treatment failure was studied in 713 paedi-atric patients presenting with acute gAS pharyngo-tonsil-litis before and after antibiotic treatment 35. Failed bacterial
eradication was demonstrated in 124 patients. The prtF1 rate was unchanged in patients treated with macrolides (9/54); in contrast it was significantly higher (p = 0.015) in strains isolated after therapy with beta-lactams (21/70) as compared to baseline isolates (6/70).
The association between resistance to erythromycin and the presence of the Fn binding protein F1 gene (prtF1) was studied in Streptococcus pyogenes isolates from 301 ger-man paediatric patients also by Haller et al. 36. The prtF1
gene was present significantly more often in macrolide-re-sistant isolates.
Streptococcal pyrogenic exotoxin (spe) genes spea and spec were studied in Streptococcus pyogenes isolates, re-covered from paediatric patients with pharyngitis, asympto-matic children and GaS-invasive disease 37. the spea gene
was detected in 13.9% of asymptomatic children, 16.8% of pharyngitis isolates and 25% of invasive cases. The spec gene incidence was higher in paediatric populations (55.4% in pharyngitis isolates and 65.8% in asymptomatic children) than in invasive isolates (30%; p < 0.0001). Mac-rolide resistance was detected in 38.0% of asymptomatic children, 37.6% of pharyngitis populations and 26.6% of invasive cases. the authors suggest that the incidence of exotoxin and antibiotic-resistance genes among populations may have no clinical significance.
31
more prtF1-positive organisms than macrolides. Macrolides are the best and first choice for the antibiotic treatment against GaS as supported by the result of a high overall
eradication rate (88%) of prtF1-positive isolates, belonging to both the erythromycin-susceptible and -resistant pheno-types 35.
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received november 10, 2006 - Accepted december 6, 2006
Address for correspondence: Prof. d. Passàli, via Anagnina 718, 00118 roma, italy - Fax +39 06 79844154 - e-mail: d.passali@virgilio.it
